1. Technical Field
The invention relates to a method of manufacturing a liquid crystal display device comprising liquid crystalline material dispersed between a first and a second substrate, the first substrate comprising a patterned layer of a polymerized material having a cholesteric order wherein the axis of the molecular helix extends transversely to the layer.
The invention further relates to a method of manufacturing a liquid crystal display device comprising a layer of a polymerized material having a cholesteric order wherein the axis of the molecular helix extends transversely to the layer and wherein the pitch of the helix shows exhibits a gradient in the direction along the axis.
The invention also relates to a liquid crystal display device, such as one obtainable by one of the methods above.
2. Description
A method as mentioned in the opening paragraph is known per se. For example, United Kingdom patent specification GB 2,314,167 describes a liquid crystal (LC) display for stereoscopic vision comprising a patterned layer of a cholesterically ordered material. In accordance with this patent specification, such a layer may be manufactured by first providing a uniform layer of a cholesteric material on a substrate. By polymerizing areas of this layer at different temperatures, a patterned cholesteric layer is obtained. Use is made of the fact that the pitch of the molecular helix of the cholesterically ordered material is temperature-dependent. By polymerizing areas of the layer at a given temperature, the pitch associated with this temperature is, as it were, frozen in these areas.
The known method has drawbacks. For example, in practice it has been found that the known method is difficult to implement. This notably applies to the case where more than two areas having mutually different pitches must be provided in the layer. In that case, the method of manufacture is elaborate in that a relatively large number of masking steps is necessary and the precision with which the masks are adjusted is very critical. Moreover, the maximum difference in pitch which can be realized between the different areas by means of the known method appears to be relatively small. Patterning at different temperatures also appears to be difficult in practice.
It is an object of the invention to obviate these drawbacks. More particularly, it is an object of the invention to provide a method of manufacturing a liquid crystal display comprising a patterned layer of a polymerized material having a fixed cholesteric order wherein the patterned layer is manufactured using a simple and cost-effective method. The patterning step(s) of said the method should not necessarily involve the use of different temperatures and allow relatively large pitch differences between the different regions of the patterned layer to be obtained.
These and other objects achieved by a method of manufacturing a liquid crystal display device comprising a liquid crystal layer dispersed between a first and a second substrate, the first substrate comprising a patterned layer of a polymerized material having a fixed cholesteric order wherein the axis of the molecular helix extends transversely to the patterned layer and the patterned layer has at least a first and a second region in which the pitch of the molecular helix is mutually different, in which method the patterned layer of a polymerized material having a fixed cholesteric order is manufactured in accordance with a method comprising the steps of:
a. providing a layer of a polymerizable and/or crosslinkable cholesterically ordered material comprising a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducable by radiation,
b. irradiating the layer in accordance with a desired pattern so that at least in a first region the convertible compound is converted to a different extent than in a second region,
c. polymerizing and/or crosslinking the irradiated polymerizable and/or crosslinkable cholesterically ordered material to form a three-dimensional polymerized cholesterically ordered material in which the cholesteric order is fixed.
It has been found that, using the method according to the invention, the patterned layer of cholesterically ordered material can be manufactured in a simple and cost-effective manner. The method is performed at the same temperature throughout, with the maximum pitch difference between the areas being relatively large. In any case, the pitch difference is sufficient to provide the patterned layer with colored regions spanning the entire visible range thus allowing full-color LC displays to be manufactured in a simple manner.
Due to the presence of the molecular helix, the patterned layer of cholesterically ordered material has regions which each selectively reflect circularly polarized electromagnetic radiation of a band of wavelengths. The central wavelength xcex of the band of reflected wavelenghts is determined by the pitch p of the molecular helix, according to xcex=p.n, where n is the average refractive index of the cholesterically ordered material. The bandwidth xcex94xcex is given by xcex94xcex=p. xcex94n, where xcex94n is the birefringence of the uniaxially oriented phase corresponding to the cholesterically ordered phase. In the visible range, the regions selective reflect circularly polarized light of a particular color. Typically, with xcex94n being less than about 0.15 and n of the order 1, the bandwidth in the visible range of the spectrum is 60 to 90 nm.
Because the patterned layer does not absorb any radiation incident upon it, it is not only a color and circular light selective reflector, but also a filter which selectively transmits light of the opposite handedness within the reflection band. Outside its reflection band, the cholesterically ordered material is transparent and transmits both polarization components.
By (partially) converting the convertible compound in the irradiated regions of the layer, the pitch of the molecular helix in the layer, and thus the color, is altered in these regions. The difference in pitch between the first and the second region is proportional to the difference in the amount of convertible compound in the converted state and/or the non-converted state between the first and the second region.
The conversion of the convertible compound is effected by irradiation with energy in the form of, for example, electromagnetic radiation, nuclear radiation or an electron beam. Preferably said conversion is effected by means of UV radiation. Being polymerized and/or crosslinked, the cholesteric order of the pattern-wise irradiated layer of polymerizable cholesterically ordered material is fixed. Being fixed, the cholesterically ordered material has lost its liquid crystalline character and is not capable any more to respond to an electric field in a manner typical of liquid crystalline materials.
Having a fixed cholesteric order, the patterned layer is capable of withstanding high temperatures in particular those temperatures which are used during the manufacture of (other parts of) the liquid crystal display device and those temperatures typically experienced during its service life. Also, the patterned layer is resistant to prolonged UV exposure. Resistance to UV exposure is improved if the patterned cholesteric layer is cross-linked.
In U.S. Pat. No. 5,555,114 a method of manufacturing a passive matrix LC display comprising a layer of cholesterically ordered material is disclosed. The known method does not involve the use of convertible compounds to control the pitch. Also, it does not even disclose how a patterned multi-color cholesteric layer is to be manufactured.
It is to be noted that, preferably, the cholesteric layer has a low absorbance for the radiation used in step b, and the radiation intensity along the axis of the helix (i.e. transverse to the layer) is relatively constant within each region. Consequently, the irradiation dose transverse to the layer is relatively constant, and therefore the value of the pitch, viewed along the axis of the helix, is relatively constant within each of the first and second regions of the patterned layer. However, as stated above between the first and second regions the pitch may differ. Viewed in the plane of the layer, the different regions are adjacent to each other, not subjacent.
When the polymerizable cholesteric layer has a high absorbance for the radiation used in step b, the radiation intensity will show a gradient transverse to the layer according to Beer-Lambert""s law. Consequently, the top of the layer will receive more radiation than the bottom of the layer. This will lead to the formation of a gradient in the pitch, viewed along the axis of the helix (i.e. transverse to the layer). Adding an light-absorbing material in a non-absorbing cholesteric layer also yields or increases a gradient in the pitch. The gradient in the pitch gives a broadening of the reflection band according to xcex94xcex=xcex94p.n wherein xcex94p is the difference in maximum and minimum pitch of the gradient. Using pitch gradients broad-band cholesterically ordered layers can be obtained. Typically, the band may span the entire visible range. Such pitch-gradient layers may be used as broad-band circularly polarized light reflectors and/or, asno absorption of light occurs, broad-band circularly polarized light filters. Such broad-band filters and reflectors can be advantageously used in the manufacture of an LC display. Thus, in another aspect, the invention relates to a method of manufacturing a liquid crystal display device comprising a layer of a polymerized material having a fixed cholesteric order wherein the axis of the molecular helix extends transversely to the layer and the pitch of the helix has a gradient in the direction of said axis, in which method the layer of polymerized cholesterically ordered material is manufactured in accordance with a method comprising the steps of:
a. providing a layer of a polymerizable and/or crosslinkable cholesterically ordered material comprising a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducable by radiation,
b. irradiating the layer thus converting, at least partially, the convertible compound to its converted state,
c. polymerizing and/or crosslinking the irradiated polymerizable and/or crosslinkable cholesterically ordered material to form a three-dimensional polymerized cholesterically ordered material in which the cholesteric order is fixed,
wherein the absorption of the polymerizable and/or cross-linkable layer and the intensity of the radiation used in step b are selected such that the radiation is substantially absorbed by the polymerizable and/or cross-linkable layer, creating, transversely to the layer, a gradient in the pitch of the molecular helix.
The method of obtaining the layer of cholesterically ordered material is a rapid method, in particular compared to the methods disclosed in U.S. Pat. No. 5,793,456. An additional advantage of the method according to the invention is that the pitch of the cholesterically ordered material is determined by the irradiation dose instead of the irradiation intensity as in U.S. Pat. No. 5,793,456. A certain irradiation dose can be administered in a short period of time using a high irradiation intensity thus reducing the time to manufacture the layer.
By polymerized and/or crosslinked, the cholesteric order of the pattern-wise irradiated layer of polymerizable cholesterically ordered material is fixed. Being fixed, the cholesterically ordered material has lost its liquid crystalline character and is as not capable any more to respond to an electric field in a manner typical of liquid crystalline materials.
Comprising a layer having a fixed cholesteric order, the gradient-pitch cholesteric layer is capable of withstanding high temperatures in particular those temperatures which are used during the manufacture of (other parts of) the liquid crystal display device and those temperatures typically experienced during its service life. Also, it is resistant to prolonged UV exposure. Resistance to UV exposure is improved if the pitch-gradient cholesteric layer is cross-linked.
Alternatively, the method of manufacturing the cholesterically ordered layer having a pitch gradient used in the present invention may be combined with the methods described in U.S. Pat. No. 5,793,456. In said combination the effects of photo-isomerisation and diffusion are combined in order to obtain an even larger gradient in the pitch of the molecular helix.
An embodiment of the method according to claim 3 wherein the irradiation dose in accordance with step b decreases from the top to the bottom of the cholesteric layer, is characterized in that, at the bottom of the cholesteric layer, said irradiation dose is less than 0.9 times the irradiation dose at the top of said layer.
When the cholesteric layer is made to absorb the radiation used in accordance with step b, the radiation intensity will show a gradient transverse to the layer according to Beer-Lambert""s law. Consequently, the top of the layer will receive more radiation than the bottom of the layer. Said variance in the irradiation dose over the cross-section of the layer will lead to the formation of a gradient in the pitch of the molecular helix, viewed along the axis of the helix (i.e. transverse to the layer). This gradient in the pitch provides the optically active layer with a larger bandwidth, the value of which is proportional to the value of the variation in the pitch. When the cholesteric material""s absorbance of the radiation used in method step b is to small to produce the desired gradient in the pitch of the molecular helix in a certain amount of time, an absorbing material may be added to the cholesteric layer to obtain the required absorbance.
A preferred embodiment of the method according to claim 3 is characterized in that, at the bottom of the cholesteric layer, the irradiation dose in accordance with step b is less than 0.75 times the irradiation dose at the top of said layer. Said preferred variation of the irradiation dose across the thickness of the cholesteric layer yields a reflection band, which may span a substantial part of the visible spectrum.
An interesting embodiment of the method according to the invention is characterized in that irradiation in accordance with step b is performed such that the irradiation dose is different for at least a first and a second region of the layer thus obtaining a patterned cholesterically ordered layers which regions are juxtaposed, as viewed in the plane of the layer, and have a different value of the pitch of the molecular helix. The different irradiation doses may be realized using different irradiation periods at a substantially constant intensity. Alternatively, the different irradiation doses may be realized by using a higher irradiation intensity at a substantially constant irradiation period.
Pattern-wise irradiation can be performed sequentially by means of, for example, a laser or by means of a mask. Preferably, however, if more than two regions having mutually different pitch are to made, masks are used having a number of apertures which have a different transmissivity to the radiation used. Such a mask is also referred to as a grey-scale mask. It has three or more areas in which the pitch of the molecular helix is different can be obtained in one irradiation step, using one mask.
In particular a patterned layer comprising red green and blue colored regions can be manufactured using a single mask exposure instead of three masks as is done conventionally. Moreover, the multi-color patterned layer can be obtained without using any lithographic patterning step.
The use of a grey-scale mask is an invention independent of the methods of manufacturing of the present invention. In particular it can be used for any method of providing a patterned layer of cholesterically ordered layer which involves the steps a and b and not necessarily c or, if a polymerization step and/or cross-linking step is performed, the polymerization and/or cross-linking need not be such that the cholesteric order is fixed. For example, the grey-scale mask can be used to obtain a patterned anisotropic cholesterically ordered gel analogous to gels disclosed in U.S. Pat. No. 5,188,760. Also, the grey-scale mask may be used to manufacture the active color filters disclosed in U.S. Pat. No. 5,668,614.
In principle, a large number of types of convertible compounds influencing the pitch of the molecular helix of cholesterically ordered material may be used within the scope of the invention. In the first place, convertible chiral compounds are feasible, which, due to irradiation, fall apart into non-chiral compounds. The presence of chiral compounds promotes the formation of a cholesteric order in a liquid crystalline solution. Irradiation of selected areas of a cholesterically ordered layer with decomposable chiral compounds leads to an increase of the pitch of the molecular helix in these areas.
Another advantageous embodiment of the method according to the invention is characterized in that the convertible compound comprises an isomerizable, chiral compound. The different isomeric forms of such an isomerizable chiral compound usually have a different influence on the pitch of the molecular helix of the cholesterically ordered material. By locally changing the ratio of these isomeric forms by way of irradiation, the pitch is changed. This provides an elegant possibility of manufacturing patterned layers of a polymer material with a cholesteric ordering and a different pitch. To prevent diffusion of the isomerizable, chiral compound in the patterned layer, this compound is preferably bound via a chemical bond to the liquid crystalline polymer material having the cholesteric order. In the latter case, the UV stability of the patterned layer also appears to have been improved.
Further examples of suitable convertible compounds are the tunable chiral compounds disclosed in U.S. Pat. No. 5,668,614.
The polymerizable and/or cross-linkable cholesterically ordered material used in the method according to the invention comprises liquid crystalline monomers, liquid crystalline oligomers and/or liquid crystalline linear polymers with reactive groups. Due to the presence of these reactive groups, this material can be converted into a polymer material by polymerization and/or into a three-dimensional molecular network by crosslinking. For the reactive groups, notably epoxy groups, vinyl ether groups and/or thiolene groups are suitable. Particularly suitable reactive groups are those of the (meth)acrylate type. It has been found that cholesterically ordered polymer layers having a high optical quality can be obtained with these types of reactive groups. It is to be noted that, when using linear polymers, only crosslinking is necessary for obtaining a three-dimensional network. However, when monomers and/or oligomers are used, polymerization and crosslinking should take place for obtaining the envisaged three-dimensional molecular network.
In particular a suitable polymerizable cholesterically ordered material comprises nematogenic monomers and a chiral compound which renders the nematogenic monomer capable of forming a cholesteric phase. The chiral compound by itself need not be capable of forming a liquid crystalline phase.
The chiral compound may be a convertible chiral compound in the sense defined above or may be a chiral compound additional to the convertible compound.
The chiral compound may be provided with one or more reactive groups in which case it is built into the (cross-linked) polymer. In particular, the chiral compound may be and preferably is a chiral nematogenic monomer. If the nematogenic monomer and/or chiral (nematogenic) monomer comprises at least two reactive groups of the above-mentioned type the cholesterically ordered material is cross-linkable.
A preferred cholesterically ordered material comprises chiral nematogenic monomers and a chiral convertible compound. Another preferred material comprises nematogenic monomers, a (non-convertible) chiral compound and a chiral convertible compound. Yet another comprises nematogenic monomers, chiral monomers and convertible chiral monomers.
A particularly preferred cross-linkable cholesterically ordered material comprises (chiral) nematogenic monomers having two reactive groups.
The stabilization of the cholesteric layer in process step c after selective adjustment of the pitch of the cholesterically ordered layer in process step b, is an important step in the method according to the invention.
An embodiment of the method according to the invention is therefore characterized in that the polymerization and/or crosslinking is initialized and/or catalyzed by the addition of an initiator or catalyst from the fluid or gaseous phase. Said addition is preferably performed after steps a en b in accordance with the invention in order to prevent a polymerization and/or crosslinking reaction during steps a and b. Various initiators and catalysts are suitable and are well known to those skilled in the art.
A further embodiment of the method according to the invention is therefore characterized in that polymerization and/or crosslinking is induced by a thermally decomposable initiator. In that case, the layer of cholesterically ordered material preferably comprises a small quantity of a thermally decomposable polymerization initiator. Said initiator is inactive during process step b according to the invention. Subsequently, the polymerization and/or crosslinking of process step c, may be effected by activating the initiator at an elevated temperature.
An embodiment of the method according to the invention is characterized in that polymerization and/or crosslinking is effected by means of electron-beam irradiation. Very hard layers can be manufactured by means of this method. In this variant of the method according to the invention, it is not necessary to use a polymerization initiator.
An embodiment of the method according to the invention is characterized in that polymerization and/or crosslinking is effected by exposure to actinic radiation. The polymerization and/or crosslinking of a layer of the cholesterically ordered material (step c) can take place in the presence of a photo-initiator by using actinic radiation such as UV radiation. An advantage of using photo-polymerization is that this method permits local polymerization and/or crosslinking in very small areas.
Since the conversion of the convertible compound (step b) is also preferably effected by means of UV radiation, step b and step c of the method claimed may interfere with one another. To eliminate or at least substantially reduce the interference among these method steps, the next three preferred embodiments of the method, as described below, may be used:
A first preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed at a temperature, at which the polymerization and/or crosslinking reaction is substantially hampered. The polymerization and/or crosslinking reaction rate is temperature-dependent. At a low temperature (at a high viscosity), the polymerization and/or crosslinking reaction is slower than the reorientation of the molecular helix, thus allowing the pitch of the cholesterically ordered material to be adjusted with limited or substantially no polymerization and/or crosslinking taking place. At a high temperature (at a low viscosity), the polymerization and/or crosslinking reaction is faster than the reorientation of the molecular helix, thus allowing a polymerization and/or crosslinking with a limited or substantially no change in the pitch of the cholesterically ordered material. In conclusion, applying two irradiation steps, each at a different temperature, method steps b and c do not interfere with each other.
A second preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed by means of radiation which is substantially inert with respect to the initiation of the polymerization and/or crosslinking reaction. This method uses two irradiation steps, each step using radiation with a different wavelength. According to this embodiment of the method according to the invention, the layer is irradiated in step b with a desired pattern using radiation which is substantially inert with respect to the initiation of the polymerization and/or crosslinking reaction, thus allowing the pitch of the cholesterically ordered material to be adjusted with limited or substantially no polymerization and/or crosslinking. Subsequently, the polymerization and/or crosslinking is effected by means of actinic radiation having a suitable wavelength to initiate polymerization and/or crosslinking. Preferably, but not necessarily, said actinic radiation has a wavelength with a limited or substantially no influence on the pitch of the cholesterically ordered material. If said actinic radiation induces an additional change in the pitch of the cholesterically ordered material, then this must be taken into account when setting the pitch of the cholesterically ordered material in process step b.
A third preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed in an atmosphere which substantially hampers the polymerization and/or crosslinking reaction. This method uses two irradiation steps, each step being performed in a different atmosphere. According to this embodiment of the method according to the invention, the layer is irradiated in step b with a desired pattern in an atmosphere comprising molecules, e.g. oxygen or nitrogen-monoxide, that quench the activated photo-initiators. Said quenching essentially deactivates the activated photo-initiators and thereby hampers the polymerization and/or crosslinking reaction. Consequently, the pitch of the cholesterically ordered material can be adjusted with limited or substantially no polymerization and/or crosslinking. Subsequently, the polymerization and/or crosslinking in accordance with step c is initiated by irradiation in a non-quenching atmosphere, e.g. nitrogen. Preferably, but not necessarily, the second wavelength used for the irradiation in accordance with step c, has a limited or substantially no influence on the pitch of the cholesterically ordered material. If said actinic radiation induces an additional change in the pitch of the cholesterically ordered material, then this must be taken into account when setting the pitch of the cholesterically ordered material in process step b.
The invention also relates to a liquid crystal device obtainable by a method in accordance with the invention.
The invention further relates to a liquid crystal display device. In accordance with the invention it is a liquid crystal display device comprising a liquid crystal layer dispersed between a first and a second substrate, the first substrate comprising a patterned layer of a polymerized and/or cross-linked material having a fixed cholesteric order wherein the axis of the molecular helix extends transversely to the patterned layer, wherein the patterned layer comprises a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion being inducable by radiation and wherein the patterned layer has at least a first and a second region in which the pitch of the molecular helix is mutually different.
The liquid crystal device comprising the layer of polymerized material of fixed cholesteric order in accordance with the invention or obtainable by the method in accordance with the invention, can be manufactured in a simple and cost-effective manner thus reducing the cost of the display device. Due to the presence of the convertible compound the patterned cholesteric layer can be obtained at one and the same temperature and patterning does not require any lithographic patterning step. If the layer has gradient pitch the layer can be manufactured in a short time thus reducing the cost of the device. The patterned layer comprising polymerized and/or cross-linked material is resistant to the processing temperatures involved in manufacturing the LCD and also resistant to ambient temperatures during its service life. The LCD device is UV-resistant. Cross-linking improves the UV-resistance even further.
Furthermore, the patterned cholesteric layer being reflective instead of absorbing and thus in principle 100% efficient, the liquid crystal device can be made highly efficient in terms of light output.
Additional advantages associated with LCD devices in accordance with the invention which are of the reflective type, are that the contrast of the LCD is improved due to the polarization sensitivity of the cholesteric layer, the color purity and reflectivity properties can be selected more independently from one another, and the microstructure of the cholesterically ordered layer can be selected such that the layer reflects diffusively instead of specular. Thus the reflector of the LCD can be of a much simpler structure compared to the complicated reflector structure of LCD devices comprising absorbing color filters.
In a preferred embodiment of the liquid crystal device, the patterned layer has first, second and third regions selectively reflecting red green and blue colored light respectively.
The LC device comprising such a full-color patterned layer is particularly simple to manufacture as the patterned layer can be obtained using a single exposure with a grey-scale mask having regions of three different transmissivities.
In another aspect, in fact an aspect which is wider applicable than the liquid crystal devices comprising the cholesterically ordered layers in accordance with the invention, the invention relates to reflective active matrix liquid crystal display device comprising a liquid crystal layer dispersed between an active and a passive substrate plate, the active substrate plate being present on the side opposite the viewing side and comprising a plurality of optically transparent pixel electrodes and a plurality of active switching elements for controlling the voltage supplied to said pixel electrodes, which switching elements are spatially separated from, positioned subjacent and electrically connected to said plurality of pixel electrodes, wherein the space separating the switching elements and the pixel electrodes is filled with an electrically insulating color selection layer which selectively reflects light of particular wavelengths and renders the switching elements invisible to the light reflected.
The LC device according to this aspect of the invention has a fill-factor of 85-90% compared to 70-80% for conventional LC devices. The fill-factor is the percentage of the display area actually contributing to light-emission. The large fill-factor is, on the one hand, due to the fact that the active switching elements are positioned subjacent the pixel electrodes and are rendered invisible to the viewer by the color selection layer. On the other hand, the color selection layer and the pixel electrodes being located on the same plate, removes the need for accurate alignment of the active and passive plate with respect to each other. Thus, tolerances can be tightened and less non-emissive area is present between the pixels. Conventionally, the passive and active plate need to be accurately aligned because the color selection layer and pixel electrodes are located on separate plates.
Furthermore, as the color selection layer is also used as the layer which insulates the switching elements from the pixel electrodes to prevent short-circuits, a compact liquid crystal device is obtained.
If appropriate, the liquid crystal device further comprises a light-absorbing layer for absorbing any light transmitted by the color selection layer. In order to make the switching elements less visible with respect to light transmitted by the color selection layer, the light-absorbing layer is preferably located between switching elements and the color selection layer.
The advantages associated with the LC devices described above which have a color selection layer arranged between the active switching elements and the pixel electrodes are not exclusively obtained when combined with the patterned cholesterically ordered layers used in the present invention but are obtained using any color selection layer capable of selectively reflecting light of a particular color such that the underlying switching element is masked, that is invisible to the viewer.
An example of such a color selection layer is a diffusely reflecting light-absorbing layer containing conventional dyes and/or pigments. Alternatively, the dyes and/or pigments may be replaced with photoluminescent, fluorescent and/or phosphorescent compounds of a conventional type. An example of a non-absorbing color selection layer is a (conventional) polymer dispersed liquid crystal or any other layer having two phases, one dispersed in the other, which mutually differ in refractive index. The color selection layers may or may not in addition to or instead of reflecting selectively a particular color, selective reflect a particular polarization component of the incident ambient light. If not, the color selection layer may be combined with a polarizer.
Instead of a color selection layer, the electrically insulating may also be a broad-band polarizer, such as a broad-band reflective polarizer, such as a broad-band cholesteric polarizer.